This invention relates to a slide blank, a slide and a process for producing a slide. The term "slide blank" is used herein to refer to a unit which resembles a slide lacking an image, and which upon imaging will form a ready-mounted slide suitable for projection.
Hitherto, slides have typically been produced by exposing a roll of silver halide film using either a camera or a film recorder (for example that sold as the CI-5000 film recorder by Polaroid Corporation), which receives a digital image from a computer or similar image processing equipment and exposes the film. In either case, only a latent image is produced upon the film, which requires development and fixing to produce visible images. After development and fixing, the various images on the film are separated from one another and each imaged film portion is mounted by placing it in a slide mount. Conventional slide mounts typically consist of two rectangular sheets of plastic, card or other relatively rigid material, each sheet having a rectangular central cut-out or "window." The developed and fixed film portion is sandwiched between the two sheets of the slide mount so that its image can be viewed in transmission through the two windows, which are aligned with each other and with the image, and the two sheets of the slide mount and the film portion are all secured together.
Such conventional slides suffer from several discrete problems, most of which are felt acutely by users making presentation graphics slides. As with any silver halide roll film, each roll of slide film can produce a number (typically 12, 20, 24 or 36) of images, and one must either expose the whole roll before processing or waste the unexposed portion of the roll. In addition, the development and fixing of the latent images require substantial investment in processing equipment, or the delays inherent in the use of independent photographic processors. Even those who regularly produce presentation graphics slides, and have "in-house" access to film recorders, typically rely upon such processors to develop and fix the film, thus incurring delays of a few hours to a day between the exposure of the film and the availability of the finished slide.
Polaroid Corporation sells, under the Registered Trade Mark "POLACHROME," slide films comprising diffusion transfer film units ("instant films"); these slide films, and apparatus for their processing, are described, for example, in Liggero et al., The Polaroid 35 mm Instant Slide System, J. Imaging Technology, 10, 1-9 (1984), and Sturge, J., Walworth, V., and Shepp, A. (eds.), Imaging Processes and Materials (Neblette's Eighth Edition), Van Nostrand Reinhold, New York (1989), pages 194-95 and 210-11. These slide films comprise a plurality of photosensitive elements, which are exposed in the same manner as conventional silver halide films. After as many of the photosensitive elements as desired have been exposed, the whole film is run through a specially designed apparatus, which causes development and formation of images on image-receiving elements. The image-receiving elements are then peeled from the photosensitive elements, separated from one another and mounted in the same manner as conventional slide films. Although this type of slide film does eliminate the delays inherent in the processing of conventional slide films, it still requires that all the photosensitive elements in a film be exposed before any are developed, or the remainder wasted, and the mounted slides produced are similar to those produced from conventional slide films, and thus suffer from the disadvantages of conventional mounted slides discussed below.
Conventional slides also suffer from problems associated with the physical form of the finished slide. It is not easy to secure the film portion securely between the two parts of the slide mount in a manner that will prevent movement of the film portion during heavy use of the slide, such as may occur when the slide is used for repeated presentations or in an automated slide changer at an exhibition. Even slight movement of the film portion relative to the slide mount causes an objectionable strip of white to appear along one edge of the projected image. Furthermore, in a conventional slide the fragile film portion is exposed through the windows in the slide mount and is easily damaged or marked, for example by the fingerprints of a user during handling. Furthermore, the heating which the exposed, relatively flexible film portion undergoes during projection tends to cause the film portion to buckle out of the focal plane of the projector lens, and such buckling may adversely affect the quality of the projected image. To prevent or reduce such marking or buckling, so-called "glass mounts" are sometimes used. These glass mounts resemble conventional slide mounts but sandwich the film portion between two thin, transparent sheets of glass, which extend across the windows in the slide mount. Although glass mounts do reduce the risk of accidental marking or buckling of the film portion, the glass sheets are themselves fragile and are readily broken. In addition, dirt or other particles can become trapped between the glass sheets and the film portion, causing unwanted artifacts on the image seen when the slide is projected.
Whether or not glass mounts are used, the difference in thickness between the window and the remaining portions of the mounted slide leaves a "step" extending around the image. This step tends to trap dirt, fibers and other detritus, which are difficult to remove without damaging the film portion, and which may produce undesirable artifacts when the slide is projected.
Conventional slides place restrictions on the shape of the images that can be produced. Slide mounts are normally only produced with windows having a fixed aspect ratio, and the image must either conform to this aspect ratio or part of the window must be covered by an opaque area, thus reducing the size of the image seen upon projection. Obviously, if desired, images can be produced in either portrait or landscape orientation, but if a presentation includes slides in both orientations, the user must manually place the slides in the projector in their correct orientation, and most frequent users of slides are familiar with the embarrassment that results when a slide is inadvertently shown in the wrong orientation.
Perhaps the worst disadvantage of conventional slides, however, is the lack of any facility for keeping one or more identifying indicia (for example, time and date of production, number of the slide in a series, or the name of the data file used to produce the image) associated with the image and visible on the mounted slide. Cameras are known having backs that can place the time and date, or other user-defined indicium, on a small area of a negative as it is exposed, so that a reflection print produced from the negative will display the indicium, usually in an inconspicuous corner of the print. Provision of such a visible indicium is not practical in the case of slides, since the user needs to be able to read the indicium on the slide before he places it in the projector, and an indicium large enough to be legible in these circumstances would occupy so large a proportion of the slide as to be highly objectionable when projected. Although it is possible to provide appropriate indicia on mounted slides by writing, printing or securing adhesive labels on the surface of the slide mount, there remains the difficulty of matching up the indicia with each slide after the slide has been returned from processing. This problem is especially difficult for frequent users of presentation graphic slides, who may have several sets of slides being processed at any one time, and may have several slides of the same general type (for example, pie charts), or several revisions of the same slide, which are easily confused and thus subject to mislabeling. The risk of mislabeling is increased by the ease with which the order of a series of slides may be disturbed by the many handling operations needed in conventional processing.
One commercial form of slide mount attempts to overcome this problem by providing a small cut-out on one half of the slide mount adjacent its window, this cut-out serving to expose a non-image area of the film so that any indicia on this non-image area can be read in reflection against a background provided by the other half of the slide mount. When such a slide mount is used with a conventional silver halide film, the non-image area exposed is that containing one set of the sprocket holes of the film, and conventional cameras and film recorders will not print in this area. Furthermore, the area available is extremely limited, since the edge of the film must be secured in the slide mount, and the area available is interrupted by the sprocket holes themselves. In practice, the only indicium which can be visible in the cut-out is the frame number of the image on the film, and while the use of such a slide mount serves to prevent placing a series of slides in the wrong order, the user is still left with the problem of associating each frame number with the appropriate caption or other indicium. Moreover, the visible frame numbers do not assist the user in identifying the roll of film from which the slide is derived.
Use of slides in presentations would be greatly simplified if a system could be developed by which a caption or other identifying indicium could be associated with an image as it is created (normally by means of computer software) such that a slide produced from the image would display the caption in a legible size on the slide mount outside the window.
In recent years, various "direct-imaging media" have been developed which allow direct formation of a visible positive image on the medium without requiring development or fixing steps using liquid reagents. Such media include those described in U.S. Pat. Nos. 4,602,263; 4,720,449; 4,720,450; 4,745,046; 4,818,742; 4,826,976; 4,839,335; 4,894,358 and 4,960,901 (in which heating of the medium causes a chemical and color change in a thermally sensitive material) and the media described in U.S. Pat. Nos. 5,278,031; 5,286,612; 5,334,489 and 5,395,736, and application Ser. No. 08/141/852 (filed Oct. 22, 1993) (which media when exposed to radiation generate acid which changes the color of an indicator dye). These two types of medium may hereinafter be called "direct-imaging single sheet media."
U.S. Pat. No. 5,234,886, issued Aug. 10, 1993 on application Ser. No. 07/722,810 filed Jun. 28, 1991, describes a slide blank intended for imaging by dye diffusion thermal transfer. This slide blank comprises a rectangular piece of dye receiving material secured in the aperture of a conventional plastic slide mount. Although this slide blank can be imaged and displayed immediately after imaging without any post-imaging mounting steps, it is not very efficient for mass production, since it requires insertion and securing of individual pieces of dye receiving material within the apertures in the slide mounts, and does nothing to solve the problem of associating identifying indicia with each slide. Furthermore, slides produced from such slide blanks may suffer from certain problems often associated with dye diffusion thermal transfer images, such as the tendency for the image dye (which is present on one external surface of the slide) to release dye on to, and thus contaminate, any objects, for example slide pockets, which come into contact with the image. Such dye release is also likely to degrade the image on the slide.
As mentioned above, direct-imaging single sheet media have the advantage that no development or fixing steps requiring liquid reagents are required after imaging. Accordingly, it is not necessary for the color-forming layers of such media to be exposed on a external surface of the medium; the color-forming layers, which tend to be rather fragile, can be protected by a protective layer (also called an "overcoat") and imaged by radiation passed through this protective layer. Accordingly, it might be thought that a slide blank could be produced simply by sandwiching a direct-imaging single sheet medium between two similar sheets of plastic material to form a slide blank which would, after imaging, produce a slide closely resembling a conventional slide. Applicants have attempted to produce slides using this type of slide blank (hereinafter called a "symmetric blank"), but have discovered that such a slide blank suffers from certain mechanical problems. In such a symmetric blank, the direct-imaging medium is normally the weakest layer of the blank, and is thus the point at which delamination of the various layers of the blank is likely to begin. Placing the weak imaging medium between two substantially rigid plastic sheets renders the symmetric blank and a slide produced therefrom susceptible to accidental or deliberate delamination. Furthermore, as discussed below, the most cost-effective process for producing a slide blank involves severing individual slide blanks from large sheets or webs, preferably by die cutting, and a weak imaging medium sandwiched between two substantially rigid plastic sheets is likely to be damaged by such die cutting.
A symmetric slide blank also suffers from optical problems during imaging. During such imaging, a beam of radiation must be focussed through one of the plastic sheets and brought to a focus in, or very closely adjacent, a color-forming layer which is typically only a few microns thick. Thus, a small change in the position of the focus may prevent imaging of the color-forming layer, or at least severely reduce the image density. Unfortunately, all commercial plastic sheets suffer from substantial variations in thickness ("gauge variations"), such variations typically being .+-.10%. If a symmetric blank is produced by sandwiching an imaging medium between two 20 mil sheets, a .+-.2 mil variation in the thickness of the sheet through which exposure is effected will produce a change in the position of the focus likely to be large enough to prevent imaging of the color-forming layer. Although techniques (such as effecting a focus series on each slide) do exist for correcting the position of the focus, the use of such correction techniques adds complexity to the apparatus used to image the slide, slows down the imaging process and results in undesirable markings on the printed slide. Moreover, in a symmetric blank, birefringence is likely to be a problem. Biaxial birefringence distorts the shape of the spot produced by a focussed beam, and in extruded sheets of plastic, such birefringence varies in orientation from point to point particularly in widely separated parts of a long web, between different webs, and between slides fed into a printer in different orientations. If focus correction techniques are attempted in a material of varying birefringence, such techniques will not work at every point on every slide. Accordingly, a symmetric blank is limited to materials having low birefringence.
Finally, applicants have discovered that upon prolonged projection of slides produced from symmetric blanks, the colors of the slide tend to change, and the contrast between regions of minimum and maximum density (D.sub.min and D.sub.max regions respectively) tends to diminish. It is believed, although the present invention is in no way limited by this belief, that the reason for these undesirable changes in such slides upon prolonged projection is the large quantities of heat generated within the slide caused by absorption of radiation from the projector, and consequent unwanted development of color in non-imaged regions of the color-forming layers. For example, in a multicolor slide of this type there will normally be three color-forming layers superposed on one another. If in a particular region one of these color-forming layers is imaged to D.sub.max whereas an adjacent color-forming layer is at D.sub.min (i.e., is unimaged), during prolonged projection of the slide, large amounts of heat will be generated by absorption of projector radiation in the D.sub.max layer, and this heat generation may cause development of unwanted color in the supposedly D.sub.min layer, thus leading to a change in color in this region.
The present inventors have found that these mechanical, optical and discoloration problems are essentially eliminated by forming an asymmetric slide blank, in which the color-forming layer is or layers are kept within a limited distance of an external surface of the slide blank, and the present invention is directed to such a slide blank, the slide produced therefrom and an imaging process using such a slide blank.